Preparatory to attempting mathematical modelling of the endemic maintenance of La Cross virus (LAC) (California encephalitis group), we are attempting to quantify the factors that affect vertical (transovarial) transmission in the mosquito vector, Aedes triseriatus and the factors that affect horizontal amplification in the vertebrate hosts, chipmunks and tree squirrels. Vertical factors receiving study include: 1. maternal vertical transmission rate of LAC virus in Aedes triseriatus; 2. effects of infection on the net replacement rate of transovarially infected female A. triseriatus; 3. efficiency of horizontal transmission of transovarially infected females; 4. paternal vertical transmission rate; 5. number of generations of A. triseriatus annually, and 6. the effect of serial hatching of A. triseriatus eggs on the seasonal vertical dilution rate of LAC virus. Horizontal parameters being studied include several related to A. triseriatus population dynamics and bloodfeeding behavior.